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          <h1 class="post-title" itemprop="name headline">复合Lambda表达式</h1>
        

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        <div id="vip-container"><h2 id="1、比较器复合"><a href="#1、比较器复合" class="headerlink" title="1、比较器复合"></a>1、比较器复合</h2><p><img src="https://img-blog.csdnimg.cn/20190515201329740.png" alt="在这里插入图片描述"><br>上面是JDK8中java.util.Comparator接口，相比jdk1.7增加了好多方法，也许你会觉得奇怪，为什么接口中还能定义方法，原因是JDK8中，可以为接口添加默认实现，使用default关键字定义。</p>
<p>例如，我们可以这样定义一个比较器：<br>Comparator&lt; Apple&gt;  c = Comparator.comparing(Apple::getWeight());<br>其等价为：<br>Comparator&lt; Apple&gt;  c = Comparator.comparing(  (a) -&gt; a.getWeight()  );</p>
<p>为什么可以这样写呢？因为Comparator定义了如下静态方法：<br><img src="https://img-blog.csdnimg.cn/20190515202531959.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70" alt="在这里插入图片描述"></p>
<h4 id="1-1-逆序"><a href="#1-1-逆序" class="headerlink" title="1.1 逆序"></a>1.1 逆序</h4><p>Comparator定义了一个静态方法，reversed，故我们不需要重新再定义一个比较器，我们可以这样就能实现逆序排序：<br>List&lt; Apple&gt; apples = new ArrayList&lt;&gt;();<br>apples.sort(  Comparator.comparing(Apple::getWeight()).reversed()  );</p>
<h4 id="1-2-比较器链"><a href="#1-2-比较器链" class="headerlink" title="1.2 比较器链"></a>1.2 比较器链</h4><p>如果要支持多重排序呢？例如先根据苹果的重量，如果重量相同就按照颜色排序，那如何来实现呢？<br>apples.sort(  Comparator.comparing(Apple::getWeight()).thenComparing(  Apple::getColor()  ) );<br>之所以可以使用上述表达式，是因为Comparator定义了如下方法：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">default &lt;U extends Comparable&lt;? super U&gt;&gt; Comparator&lt;T&gt; thenComparing( Function&lt;? super T, ? extends U&gt; keyExtractor)</span><br><span class="line">&#123;</span><br><span class="line">    return thenComparing(comparing(keyExtractor));</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>温馨提示：Comparator中定义很多thenComparing重载方法,在具体使用过程中，可以先看看其函数声明。</p>
</blockquote>
<span id="more"></span>

<h4 id="2、谓词复合"><a href="#2、谓词复合" class="headerlink" title="2、谓词复合"></a>2、谓词复合</h4><p>提到谓词复合，我们就不得不提Predicate&lt; T &gt;函数式编程接口，其类图如下所示：<br><img src="https://img-blog.csdnimg.cn/20190515203712938.png" alt="在这里插入图片描述"></p>
<ul>
<li>and：与</li>
<li>negate：非</li>
<li>or：或</li>
</ul>
<blockquote>
<p>温馨提示：and 和 or 方法是按照在表达式链中的位置，从左向右确定优先级的。因此， a.or(b).and(c) 可以看作 (a || b) &amp;&amp; c 。</p>
</blockquote>
<p>使用示例：从苹果列表中找出所有红色的，并且重量超过150的苹果：</p>
<p>apples.filter(   (a -&gt; “red”.equals(a.getColor()) ).and(  a -&gt; a.getWeight() &gt; 150  )   );</p>
<p>a -&gt; “red”.equals(a.getColor())  是 (Apple a ) -&gt; “red”.equals(a.getColor())的简写。</p>
<h2 id="3、函数复合"><a href="#3、函数复合" class="headerlink" title="3、函数复合"></a>3、函数复合</h2><p>函数复合，其对应的函数式编程接口为Function&lt;T,R&gt;，其类图如下：<br><img src="https://img-blog.csdnimg.cn/20190515204237920.png" alt="在这里插入图片描述"></p>
<ul>
<li>addThen<br>andThen 方法会返回一个函数，它先对输入应用一个给定函数，再对输出应用另一个函数。<br>例如：<br>Function&lt;Integer, Integer&gt; f = x -&gt; x + 1;<br>Function&lt;Integer, Integer&gt; g = x -&gt; x * 2;<br>Function&lt;Integer, Integer&gt; h = f.andThen(g);<br>int result = h.apply(1);  // 其结果返回4，类似与数学公式  f(g(x))。</li>
<li>compose<br>先把给定的函数用作 compose 的参数里面给的那个函数，然后再把函数本身用于结果。与addThen的函数应用方向相反，同样举例说明如下：<br>Function&lt;Integer, Integer&gt; f = x -&gt; x + 1;<br>Function&lt;Integer, Integer&gt; g = x -&gt; x * 2;<br>Function&lt;Integer, Integer&gt; h = f.compose(g);<br>int result = h.apply(1);  // 其结果返回3，类似与数学公式  g(f(x))。</li>
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      var entries = [];
      var $visitors = $(".leancloud_visitors");

      $visitors.each(function () {
        entries.push( $(this).attr("id").trim() );
      });

      query.containedIn('url', entries);
      query.find()
        .done(function (results) {
          var COUNT_CONTAINER_REF = '.leancloud-visitors-count';

          if (results.length === 0) {
            $visitors.find(COUNT_CONTAINER_REF).text(0);
            return;
          }

          for (var i = 0; i < results.length; i++) {
            var item = results[i];
            var url = item.get('url');
            var time = item.get('time');
            var element = document.getElementById(url);

            $(element).find(COUNT_CONTAINER_REF).text(time);
          }
          for(var i = 0; i < entries.length; i++) {
            var url = entries[i];
            var element = document.getElementById(url);
            var countSpan = $(element).find(COUNT_CONTAINER_REF);
            if( countSpan.text() == '') {
              countSpan.text(0);
            }
          }
        })
        .fail(function (object, error) {
          console.log("Error: " + error.code + " " + error.message);
        });
    }

    function addCount(Counter) {
      var $visitors = $(".leancloud_visitors");
      var url = $visitors.attr('id').trim();
      var title = $visitors.attr('data-flag-title').trim();
      var query = new AV.Query(Counter);

      query.equalTo("url", url);
      query.find({
        success: function(results) {
          if (results.length > 0) {
            var counter = results[0];
            counter.fetchWhenSave(true);
            counter.increment("time");
            counter.save(null, {
              success: function(counter) {
                var $element = $(document.getElementById(url));
                $element.find('.leancloud-visitors-count').text(counter.get('time'));
              },
              error: function(counter, error) {
                console.log('Failed to save Visitor num, with error message: ' + error.message);
              }
            });
          } else {
            var newcounter = new Counter();
            /* Set ACL */
            var acl = new AV.ACL();
            acl.setPublicReadAccess(true);
            acl.setPublicWriteAccess(true);
            newcounter.setACL(acl);
            /* End Set ACL */
            newcounter.set("title", title);
            newcounter.set("url", url);
            newcounter.set("time", 1);
            newcounter.save(null, {
              success: function(newcounter) {
                var $element = $(document.getElementById(url));
                $element.find('.leancloud-visitors-count').text(newcounter.get('time'));
              },
              error: function(newcounter, error) {
                console.log('Failed to create');
              }
            });
          }
        },
        error: function(error) {
          console.log('Error:' + error.code + " " + error.message);
        }
      });
    }

    $(function() {
      var Counter = AV.Object.extend("Counter");
      if ($('.leancloud_visitors').length == 1) {
        addCount(Counter);
      } else if ($('.post-title-link').length > 1) {
        showTime(Counter);
      }
    });
  </script>



  

  

  
  

  

  

  

</body>
</html>
